Interferometers, based on the Michelson principle, using beam splitters and making use of a computer to process a Fourier transform of the measurements have been employed in the infra red region through the visible region to the near ultra violet region of the spectrum. The conventional Michelson interferometer has a wedge-shaped single beam splitting plate, a single wedge-shaped compensation plate (both being of very small wedge angle), and a pair of spaced apart angled (usually orthogonally disposed) plane mirrors. The normal mode of use is to move one of the mirrors of the pair along the beam at a prescribed velocity whilst detecting the optical interference fringes which appear in the output beam. The inverse Fourier transform of the detected results then recovers the spectrum. However, it is necessary to achieve and maintain a precise alignment of the mirrors so that the departure from parallelism of the recombined wave fronts is nowhere more than a prescribed fraction of the wavelength (usually one-quarter) of the radiation being examined.
Some of the problems associated with the above-mentioned difficulty have been avoided in more recent times by employing retro-reflectors (i.e. cube-corner reflectors or the so-called cat's eye reflectors) to return the split beams for recombination in place of the previously used plane mirrors. These enable a precisely parallel alignment of the incident and reflected beams of the reflectors to be automatically maintained irrespective of the angular alignment of the respective reflector to the incident beam, even during displacement of the movable reflector. However, the consequence of using retro-reflectors is that there is a lateral displacement of the reflected beam relative to the incident beam and therefore recombination takes place at a different site to that at which the beam is split. Moreover the passage of the beam through the substrate shears the beam in such a way that the separation between incoming and outgoing beams is different for the two retro-reflectors, increasing the likelihood of wave front errors.
An alternative arrangement uses two spaced apart reflectively coated plates to split and recombine the beams. These may be given matched wedge angles to deal with the satellite fringes. If the two halves have reflecting coatings on opposite faces, the reflected beam and the transmitted beam are both treated optically in exactly the same way and no compensating plate is required.
The shear problem is dealt with for small angles of incidence by offsetting one plate along the axis of the beam relative to the other plate a distance of approximately one nth of its thickness (n being the refractive index of the substrate). However the reflective surfaces of the two plates must be maintained parallel to extremely fine tolerances for the interferometer to work at all, that is parallelism must be maintained to a small fraction of the shortest wavelength of radiation to be examined, and achieving this satisfactorily and preserving the setting over long periods is made more difficult by the aforesaid axial displacement of the plates. The problem becomes more difficult to overcome as the wavelength of the radiation reduces. Satisfactory resolution down to the 250 nm region of the spectrum has been achieved but the difficulty of achieving and maintaining the necessary very fine mechanical settings begin to impinge seriously at shorter wavelengths than this.